Photopolymerizable printing plates are known for use in making flexographic printing forms. Generally, characters or images can be produced on the printing surface by exposing the printing plate to actinic radiation through an image bearing transparency. The areas which are exposed to the radiation photopolymerize, and as a result are harder and less soluble than the unexposed areas. Subsequently, the unexposed, non-polymerized areas of the printing plate can be removed, generally by use of a suitable solvent. The photopolymerized areas then remain and are in the form of the image exposed by the radiation treatment. Examples are found in the following patents: GB 1366769, U.S. Pat. No. 4,266,005, U.S. Pat. No. 4,320,188, U.S. Pat No. 4,126,466, U.S. Pat. No. 4,430,417, U.S. Pat. No. 4,460,675 and U.S. Pat. No. 5,213,948.
Such photopolymerizable printing plates usually comprise a support, an optional adhesive layer or other underlayer, one or more photopolymerizable layers, an optional elastomeric intermediate layer and a cover layer.
A generally preferred method for making such multilayer photopolymerizable printing plates is by a process in which a previously extruded photopolymerizable composition is fed into the nip of a calendar and is calendered between a support layer and a cover layer, thus forming a photopolymerizable layer between them. EP 0084851 A disclosed a preparation method for a multilayer photopolymerizable printing plate, having an added elastomeric layer between the cover layer and the photopolymerizable layer.
The photopolymerizable layers can contain polymeric binders, photopolymerizable monomers, photo-initiators, and added auxiliaries such as plasticizers, fillers, stabilizers etc.
It is desirable for the polymers used in the flexographic printing plates to have properties which are beneficial for the preparation and use of such plates. Generally, it is preferred that printing reliefs be flexible, yet resilient, as well as soft enough for non-exposed surfaces to be removed by a suitable solution, and furthermore that they facilitate photopolymerization.
In the past, polymeric binders have been thermoplastic block copolymers as disclosed in e.g. U.S. Pat. No. 6,531,263. These are generally block copoymers of the general formula A-B-A or (AB)n or (AB)nX, comprising thermoplastic blocks A and elastomeric Blocks B, particularly linear and radial block copolymers with poly (monovinylaromatic hydrocarbon) end blocks.
Non-hydrogenated styrenic block copolymers such as styrene-isoprene-styrene (SIS) and styrene-butadiene-styrene (SBS) can be used to make UV cured, flexographic printing plates. Use of SIS block copolymers generally results in a softer printing plate. However, limitations for these SIS block polymers are that they are less reactive, more expensive and also have limited resistance to degradation by ozone. SBS block copolymers on the other hand are more reactive than SIS copolymers but have greater hardness, resulting in less flexible flexographic printing plates. Also, SBS is known to be more resistant to ozone compared to SIS polymers.
Generally mixing SIS and SBS block copolymers leads to poor results because the blends are generally thermodynamically unstable. Such incompatibility leads to haziness or the phases may even separate. Furthermore, the presence of haze interferes with the curing of the plate by scattering actinic radiation. It is preferred for such flexographic plates to be transparent so that the dispersibility of the radiation is reduced.
There exists a need for an improved block copolymer which would possess the beneficial properties of both SIS and SBS, such as flexibility, lower melt viscosity and high reactivity, while minimizing or eliminating the drawbacks of each block copolymer alone or as a mixture.
Furthermore, there exists a need for improved properties with respect to curable adhesive compositions. It would be advantageous to provide a block polymer which would have improved radiation sensitivity, as well as melt viscosity and a superior balance of properties for use in hot melt, radiation curable adhesive compositions.